Legion

A legion, also called a Roman legion, was a compartment of the Roman army, consisting of around 5,000 men each. Founded in the 6th century by Servius Tullius, the Etruscan king of Rome, the legions consisted of men from both classes (plebeian and patrician) that were raised to fight Rome's enemies. The legionaries reformed after the Battle of Allia in 390 AD, a shock defeat by Gaul, and became an effective fighting force. The legions were flexible and equipped with the basics: steel helmets, shields bearing their family emblem, short swords (gladius), and pila (javelins). They were the backbone of Rome's military until the decline of Rome in the 4th century, where Auxilia were recruited in their stead.

Etruscan influence
The Roman legion was preceded by the Etruscan League's citizen army, made up of part-time troops. The Etruscan king Servius Tullius introduced reforms in the late 6th century BC that included the census. However, one of the most important changes was the formation of the Legii. The legions were formed out of 5,000 Roman citizens drawn from both the pauper (plebians) and the wealthy (patricians), and were typically over the age of 16 and below 50. These men were supplied with basic rations, steel helmets and armor, square shields, a gladius (short sword), dagger, and pila (javelins). The legions drew heavily from Greek culture, fighting in phalanxes like the Etruscans tought them; they were in turn instructed by the Greek Cities of southern Italy.

Allia Reforms
The legions fought successfully in the Roman wars against the Etruscan League and the other Italian states following Rome's declaration of independence in 509 BC, and they fought in the traditional Greek manner for over two centuries. However, things changed after a shock defeat by the Senones at Allia in 396 BC. Following this defeat, Rome modernized its army. They formulated new tactics such as the testudo, and stopped fighting like their Greek opponents. They were made into a standing army rather than a militia defense force like they were founded. The Romans proceeded to defeat their Greek masters and reduce them to a subservient client status by the turn of the 3rd century.

Conquest of the Legions
Rome's legions fought well against their opponents, bringing down their enemies like dominoes. They first defeated the Latins, Samnites, Campanians, and their other immediate tribal neighbors. With control of Central Italy, Rome went on to conquer the south under Gaius Victor and Caius Julius subjugated the north, evicting the Umbrians from their stronghold of Ariminum and Ravenna. In 280-275 BC, they won the Pyrrhic War, resisting Greeks with elephants for the first time and beating the marvelous military adventurer Pyrrhus of Epirus. Rome went on to wreck the Etruscans and capture their cities of Caralis, Velathri, and Ariminum, and the Romans dominated Italy as a whole after winning the First Punic War in 241 BC against Carthage.

The Roman Republic's armies went on to conquer all of Western Europe, parts of Germany, all of The Balkans, Asia Minor, the Levant, Egypt, North Africa, Spain, Greece, Britannia, and the Mediterranean islands. When Rome changed into the Roman Empire, it proceeded to stave off several invasions, and the legions continued to perform well.

Fall of Legia
The Legions, effective as they were, were expensive to maintain due to the empire's large size and constant need for security against barbarians and the Parthian/Sassanid Empire. By the 4th century, the size of the Roman citizen legions was leveled by the rise of the army of auxilia. According to historian Indro Montanelli, Rome's fall was due to internal decay: "Rome's army was comprised entirely of Germans".